The goal of this project is to identify the roles of Sox5 and Sox6 in cartilage formation and maintenance from late gestation onto adulthood. The genes encode similar Sry-related transcription factors and were previously demonstrated to have essential, redundant roles in chondrogenesis in the mouse embryo. They promote formation of the cartilage templates of endochondral bones and establishment of growth plates by controlling chondrocyte differentiation at multiple steps. Since the conventional mutants generated to determine these functions die before or soon after birth, some of the roles of Sox5/Sox6 in late gestation are unknown and postnatal roles remain totally unknown. While the data obtained in embryos and strong similarities between embryonic and postnatal cartilages suggest important functions later in life, new experiments are needed to test this hypothesis. This project will specifically determine whether Sox5/Sox6 are needed for joint and articular cartilage formation, which occur in late gestation and the neonatal period, and for growth plate and articular cartilage maintenance throughout adulthood. In the latter case, failure to express Sox5/Sox6 in articular chondrocytes is hypothesized to lead to osteoarthritis. Based on the roles of Sox5/Sox6 in determining chondrocyte fate, this project will also test whether Sox5/Sox6 control expression of regulatory genes besides the extracellular matrix genes previously identified. To bypass early lethality, Aim 1 is to use the Cre-loxP system to generate mouse lines harboring Sox5/Sox6 conditional alleles. Aim 2 is to use these mice in conjunction with established Cre transgenic mice to inactivate Sox5/Sox6 specifically in chondrocytes or in subsets of chondrogenic sites to study functions from embryogenesis through adulthood. Aim 3 is to use established Cre mice or generate new ones to inactivate Sox5/Sox6 in a temporally regulated manner and thereby study postnatal functions independently of embryonic functions. Histological and molecular analyses are proposed to characterize mutant mice. Aim 4 is to use gene expression arrays and quantitative in vivo assays to obtain a detailed profile of Sox5/Sox6-related gene expression in chondrocytes. This study should significantly extend our knowledge of the roles of important chondrogenic transcription factors and should provide novel insights into the molecular mechanisms that control joint formation, maintenance of cartilage growth plate, articular cartilage formation, and osteoarthritic degeneration of articular joints.